The peak-to-average power ratio (“PAPR”), also known as peak-to-mean power ratio (“PMPR”) or peak factor, may be an important characteristic of multi-carrier transmitted signals. The peak of the signal can often be N times greater than the average power level. These large peaks may cause intermodulation distortion which can result in an increase in the error rate. These distortions are typically caused by the limitations inherent in a transmitting amplifier.
To prevent the transmitter amplifier from limiting (clipping), the average signal power is typically kept low enough to keep the signal relatively linear through the amplifier. To transmit a high power signal, a high power amplifier may be used which requires a large DC system power. A much higher power amplifier is typically used to transmit multi-carrier waveforms than for constant envelope waveforms. For example, using 64 carrier waveforms, a 40 dBm power amplifier would require about 15 dB of back off. Therefore, instead of operation at 40 dBm (10 watts) the amplifier is only capable of operating at 25 dBm (0.316 Watts). Thus to transmit at the desired 40 dBm, a 55 dBm (316 Watt) amplifier would be needed. In addition, such large power requirements may lead to associated increased space demands and heat dissipation requirements.
With the large amount of interest and activity with Orthogonal Frequency Division Modulation (OFDM), and in particular with IEEE 802.11a and 802.11g communication technology, the PAPR problem is exaggerated. The IEEE 802.11 standard with its use of complex waveforms may require highly linear RF amplifiers. Current IEEE 802.11 physical layer integrated circuits have not implemented PAPR reduction schemes. In particular, multi-tone OFDM typically requires greater than 10 dB power amplifier back-off because of a high peak-to-average power ratio. The net result of these factors may be increased DC power demand beyond that encountered with other IEEE 802.11 techniques. The effect may be less noticeable for short duty cycle signals, but can be significant for situations requiring continuous transmission of data.
OFDM, as mentioned above, is a method of transmitting data simultaneously over multiple equally-spaced and phase synchronized carrier frequencies, using Fourier transform processing for modulation and demodulation. The method has been proposed and adopted for many types of radio systems, such as wireless Local Area Networks (“LAN”) and digital audio and digital video broadcasting. OFDM offers many well-documented advantages for multi-carrier transmission at high data rates, particularly in mobile applications. Specifically, it has inherent resistance to dispersion in the propagation channel. Furthermore, when coding is added it is possible to exploit frequency diversity in frequency selective fading channels to obtain excellent performance under low signal-to-noise conditions. For these reasons, OFDM is often preferable to constant envelope modulation with adaptive equalization and is arguably less complex to implement.
The principal difficulty with OFDM, as alluded to above, is that when the sinusoidal signal of the N carriers add mostly constructively, the peak envelope power is as much as N times the mean envelope power. If the peak envelope power is subject to a design or regulatory limit then this has the effect of reducing the mean envelope power allowed under OFDM relative to that allowed under constant envelope modulation. If battery power is a constraint, as is typically the case with portable equipment such as mobile consumer appliances, and laptops, then the power amplifiers required to behave linearly up to the peak envelope power may be operated inefficiently with considerable back-off from compression. Digital hard limiting of the transmitted signal has been shown to alleviate the problem, but only at the cost of spectral sidelobe growth and consequential bit error performance degradation.
The IEEE 802.11a standard specifies an OFDM physical layer (PHY) that splits an information signal across 52 separate subcarriers to provide transmission of data at a rate of 6, 9, 12, 18, 24, 36, 48, or 54 Mbps. The 6, 12, and 24 Mbps data rates are mandatory. Four of the subcarriers are pilot subcarriers that the system uses as a reference to disregard frequency or phase shifts of the signal during transmission. A pseudo binary sequence is sent through the pilot subchannels to reduce the generation of spectral lines. The remaining 48 subcarriers provide separate wireless pathways for sending the information in a parallel fashion. The resulting subcarrier frequency spacing is 0.3125 MHz (for a 20 MHz channel with 64 possible subcarrier frequency slots).
The primary purpose of the OFDM PHY layer is to transmit Media Access Control (MAC) protocol data units (MPDUs) as directed by the IEEE 802.11 MAC layer. The OFDM PHY layer is divided into two elements: the physical layer convergence protocol (PLCP) and the physical medium dependent (PMD) sublayers.
The MAC layer communicates with the PLCP via specific primitives through a PHY service access point. When the MAC layer instructs, the PLCP prepares MPDUs for transmission. The PLCP also delivers incoming frames from the wireless medium to the MAC layer. The PLCP sublayer minimizes the dependence of the MAC layer on the PMD sublayer by mapping MPDUs into a frame format suitable for transmission by the PMD.
Under the direction of the PLCP, the PMD provides actual transmission and reception of PHY entities between two stations through the wireless medium. To provide this service, the PMD interfaces directly with the air medium and provides modulation and demodulation of the frame transmissions. The PLCP and PMD communicate using service primitives to govern the transmission and reception functions.
FIG. 1 illustrates the frame format for an IEEE 802.11a frame as in the prior art. The PLCP preamble field is present for the receiver to acquire an incoming OFDM signal and synchronize the demodulator. The preamble has 12 symbols. Ten of the symbols are short for establishing Automatic Gain Control (AGC) and the coarse frequency estimate of the carrier signal. The receiver uses the long symbols for fine-tuning. With this preamble, it takes 16 microseconds to train the receiver after first receiving the frame.
With additional reference to FIG. 2, the signal field has 24 bits, defining data rate and frame length. A reserved bit, parity bit and tail bits are included. The IEEE 802.11a version of OFDM uses a combination of binary phase shift keying (BPSK), quadrature PSK (QPSK), and quadrature amplitude modulation (QAM), depending on the chosen data rate. The length field identifies the number of octets in the frame. The signal symbol field is convolutionally encoded and sent at 6 Mbps using BPSK no matter what data rate the signal field indicates. The convolutional encoding rate of the signal symbol is 1/2.
With additional reference to FIG. 3, the service field has 16 bits, with some bits as zeros to synchronize the descrambler in the receiver, and the remaining bits being reserved for future use (and set to zeros). The PLCP service data unit (PSDU) is the payload from the MAC layer being sent. The pad field contains at least six bits, but it is actually the number of bits that make the data field a multiple of the number of coded bits in an OFDM symbol (48, 96, 192, or 288).
With IEEE 802.11a OFDM modulation, the binary serial signal is divided into groups (symbols) of one, two, four, or six bits, depending on the data rate chosen, and converted into complex numbers representing applicable constellation points.
Various conventional approaches address the PAPR for OFDM packets. For example, U.S. Patent Application Publication 2005/0089116 to Moffatt et al. (and assigned to Harris Corporation of Melbourne, Fla. the assignee of the present invention) describes a predictive signal producing method that effectively levels transmitter output power in a multi-carrier communication system and results in approaching amplifier performance normally associated with constant carrier waveforms. The disclosed approach offers >10 dB reduction in the peak-to-average power required to support the transmission of OFDM modulation techniques.
Other approaches are described in U.S. Pat. No. 7,496,028 to Jung et al., U.S. Pat. No. 7,301,891 to Park et al., and U.S. Pat. No. 6,925,128 to Corral.
However, there may be a desire to further address the PAPR.